For Cycle 5, in addition to the dual polarization (XX, YY) and single polarization modes (XX), observations to measure the full intrinsic polarization (XX, XY, YX and YY) of sources will also be offered for 12-m Array TDM and FDM observations in Bands 3, 4, 5, 6 and 7. Only linear polarization is guaranteed to meet the ALMA specification and only within one third of the primary beam. While PIs will receive data that will allow them to generate circular polarization data, the quality and/or accuracy of that data at this time is not assured, and such data should not be used for scientific purposes.

When a Dual Polarization setup is used, separate spectra are obtained for the cross-correlated parallel hands (XX and YY). These will give two largely independent estimates of the source spectrum that can be combined to improve sensitivity.

In a Single Polarization mode, only a single input polarization (XX) is recorded. For a given resolution, this provides √ 2 worse sensitivity than the Dual Polarization case, but one can use either a factor two more bandwidth for the same spectral resolution (unless the maximum bandwidth was already being used) or a factor of two better spectral resolution for the same bandwidth.

Full Polarization measurements are non-standard, limiting the total time available for such observations. (see "What is the difference between a standard and a non-standard observing mode for Cycle 5?") Sources must have a user-specified largest angular structure that is less than one-third of the 12-m Array primary beam at the frequency of the planned observations. The expected minimum detectable degree of polarization is 0.1% (1%) for compact sources and 0.3% (3%) for extended sources for TDM (FDM) observations, respectively. Full polarization is not offered for spectral scans or mosaics. The frequency settings for continuum polarization measurements can be specified by the user, but the OT supplies default setups as detailed in Table A.5 of the Proposer's Guide. For FDM mode polarization observations any frequency setting within Bands 3, 4, 5, 6 and 7 is allowed, and the spectral setup has to be the same for the polarization calibrator and the science target.

It should be noted that Full polarization observations require about 3 hours of parallactic angle coverage for proper calibration. Science Goals with properties that lead to a total observing time estimate that is less than 3 hours will have the time estimate set to 3 hours to ensure sufficient parallactic angle coverage.